1. Field of the Invention
This invention relates to automatic exposure cameras, and more particularly to a camera having programmed exposure control.
2. Description of the Prior Art
In the conventional type of program-controlled exposure camera, the brightness information Bv, film speed information Sv and full open F-number information Avo of the photographic objective are combined by a program curve based on program constants to derive both of an effective diaphragm value and an effective exposure time. In more detail, as the light value is obtained at the full open aperture, the brightness information representing Bv-Avo has to be combined with the full open F-number information Avo to derive a Bv information which is then combined with the film speed information Sv to obtain an Ev information. This Ev information leads to determine a corresponding one combination of Tv and Av according to a program curve. For this reason, the use of the program in the exposure control requires the provision of a mechanism for setting the full open F-number information Avo of the objective, thus giving a disadvantage of calling for an increase in the complexity of information input mechanism.
Another disadvantage of the conventional type program-controlled exposure camera is that since the Ev value is translated into Av and Tv values in one-to-one correspondence regardless of however large value the full open F-number of the objective is, as has been mentioned above, with the use of an objective lens having a large Avo, when the object brightness is darkened to some extent, the program tends to operate so that the diaphragm is adjusted to the full open value. In general, however, the image quality reaches a highest level when the diaphragm is closed down from the full open aperture by a few stops. If the image quality if prevailed, therefore, it is desirable to shift the program to a direction such that the diaphragm is effectively closed down as when the camera is associated with an interchangeable objective of large Avo. As has been mentioned above, however, according to the prior art, the Avo is not taken into account in determining the Av value. Thus, it has been impossible to control the program shift in accordance with the Avo information.